Immortalized avian cell lines for virus production

ABSTRACT

The present invention relates to immortalized avian cell lines suitable for production of biologicals or viruses for vaccination. In particular, the cell lines are derived from primary cells which are transformed with at least two viral or cellular genes, one of which causes cell cycle progression whereas the other interferes with innate protective mechanisms of the cell induced by dysregulated replication. The invention moreover relates to the production of said immortalized cell lines and their use for producing biologicals or viruses for vaccination.

The present invention relates to immortalized avian cell lines suitablefor production of biologicals or viruses for vaccination. In particular,the cell lines are derived from primary cells which are transformed withat least two viral or cellular genes, one of which causes cell cycleprogression whereas the other interferes with innate protectivemechanisms of the cell induced by dysregulated replication. Theinvention moreover relates to the production of said immortalized celllines and their use for producing biologicals or viruses forvaccination.

BACKGROUND

Embryonated chicken eggs still are one of the main substrates for theproduction of human vaccines. They are able to support the replicationof a wide range of human and animal viruses. This spectrum includesattenuated viruses, i.e. defective viruses that have impaired potentialto replicate in human or mammalian cells and can thus be used asvaccines. Attenuation can be generated or maintained by continuouspassage in embryonated eggs. Chicken eggs used for human vaccineproduction must be certified to be free of a defined set of viral andbacterial contamination (specific pathogen-free or SPF). SPF eggs areavailable from commercial suppliers. The broad applicability and a longinternational track record has kept this strategy alive despite cleardisadvantages:

SPF flocks of chicken and embryonated eggs are expensive and canconstitute up to 40% of the cost of vaccines. Furthermore, it isdifficult to continually maintain SPF flocks completely free ofpathogens which is evidenced by periodic outbreaks of disease in SPFflocks. A vaccine lot cannot be released until the SPF supplier verifiesthat the parental chickens for the embryonated eggs used to manufacturethe vaccine lot were completely free of any disease. This uncertaintyadds a significant cost to the preparation of these vaccines. Inpandemic situations with sudden need for a particular vaccine (e.g.influenza) the supply of SPF eggs may be severely limited. In addition,the large-scale processes for infecting eggs and maintaining virusgrowth are time consuming and sometimes inconsistent across differentvaccine batches.

With the development of cell culture techniques vaccine manufacturershave replaced embryonated eggs with isolated chicken embryonicfibroblasts. While the use of primary cell cultures improves the safetyprofile, efficiency and reliability of the manufacturing process, italso further increases costs: chicken fibroblasts are prepared from SPFeggs by mincing embryos to establish and amplify viable cells. Typicalfor primary animal cells the fibroblasts suffer senescence: the doublingtime increases with passaging and eventually all cells die. This processoccurs after about 20 passages, much earlier than for rodent or somehuman cell substrates currently used in vaccine manufacture (such asMRC-5 or WI-38). Fibroblast cultures have to be maintained in thepresence of 5-10% fetal calf serum, adding additional risk factors tothe manufacturing process. They also require a solid surface forpropagation and do not grow in suspension, a preferred state forbioreactor applications. Even with the use of multilayer cell factoriesthis substantially limits scale-up procedures. Due to the limited livespan a complete set of safety tests has to be applied for each lot ofchicken fibroblasts.

Fibroblasts are the only cell type out of the wide variety of differenttissues from a chicken embryo that proliferates well. The predominanceof fibroblasts compared to other cell types has in some cases decreasedtheoretical virus yield because in eggs typically the chorioallantoicmembrane, an epithelial cell layer, is the main site for virusamplification.

The discussed problems have contributed to severe influenza vaccineshortages in the last two years (2003 and 2004). To overcome theselimitations, a permanent cell line growing in a synthetically definedmedium, preferably in suspension or at least on carriers, would behighly desired.

Some of the viruses typically grown in chicken fibroblasts have beenadapted to certain cell lines. BHK-21 (baby hamster kidney) cellssupport the growth of various vaccinia, influenza, and rabies vaccinestrains (Drexler, I. et al., J. Gen. Virol. 79(Pt2):347-52 (1998);Gumusderelioglu M. et al., Biotechnol. Appl. Biochem. 33:167-72 (2001);Merten, O. W. et al., Adv. Exp. Med. Biol. 397:141-51 (1996)) and easilygrow in large fermenters on carriers under serum-free conditions (Pay,T. W. et al., Dev. Biol. Stand 60:171-4 (1985); Gallegos Gallegos, R. M.et al., Arch. Med. Res. 26:59-63 (1995)). For vaccinia this applies evento the highly attenuated strain Ankara (MVA) which was developed onchicken cells. The BHK-21 cell line is accepted for production ofcertain vaccines for livestock animals (Lubiniecki, A. S., BioprocessTechnol. 10:495-513 (1990)). However, the BHK-21 line does not meet thesafety requirements for human live vaccines. BHK cells havespontaneously formed, are highly tumorigenic and their history isinadequately reported.

According to the FDA, CBER Discussion from May 12, 2000 on cellsubstrates the development of “Minimally-Purified Live-Attenuated ViralVaccines and Virus-Vectored Vaccines” in neoplastic cells derived fromnaturally occurring tumors from humans and other mammals or from humancells and mammalian cells that have been transformed by unknownmechanisms is discouraged.

As an exception to the rule the VERO cell line (originating from Africangreen monkey) is allowed as a cell substrate for vaccine manufacturebased on a proven safety profile and the lack of transformed phenotypefor a defined number of passages. The cell line has been usedextensively for the manufacture of the polio and smallpox vaccines forclinical use. However, VERO cells require attachment and are amenableonly to carrier based processes.

Additionally MDCK cells (a spontaneous cell line from dog kidneyepithelium) with a described history have been applied to themanufacture of influenza virus (Tree, J. A. et al., Vaccine 19:3444-50(2001)).

More recently, triggered by the development of vector based vaccines andgene therapy approaches, new so-called designer cell lines of humanorigin are intensely discussed and included into the spectrum ofpotential cell substrates for vaccine production (Vaccines and RelatedBiological Products advisory committee, session from May 16, 2001). Newpermanent cell lines were created to provide complementing genes forrecombinant viruses that are replication-deficient outside theproduction system. However, stable introduction of the complementinggenes requires prolonged cultivation times, which either exceed thenatural limit of passage numbers available to primary cells or thetolerated limit of passage numbers for VERO cells before fulltransformation occurs.

Designer cell lines are generated in vitro with extensive documentationusing characterized genes for transformation. For example, thecomplementing genes from the E1 region of adenoviruses by themselvesexhibit transforming properties and have allowed establishment of humancell lines, for example PER.C6 (Fallaux, F. J. et al., Hum. Gene Ther.9:1909-17 (1998)). The application of these cell lines is not limited tothe viral vector they are designed for but may be extended to otherviruses. For example, influenza virus can be propagated on PER.C6 (Pau,M. G. et al., Vaccine 19:2716-21 (2001)). However, this finding does notapply to all viruses relevant to vaccine development, in particularavian viruses such as Marek's disease, infectious bursal disease,Newcastle disease, turkey herpes, or chicken anemia viruses. While someof these viruses replicate well on mammalian cell lines, virus growth isoften poor. For other viruses, replication is poor and limited toparticular especially adapted strains.

In addition, with adaptation to a primate-derived cell substrate,receptor binding sites on the virus are likely to change resulting in amodified antigen pattern and thus a general effect on immunogenicity.This genetic adaptation may reverse attenuation for strains which havebeen developed via passaging in avian cells such as MVA orchicken-adapted measles virus (Escoffier, C., Gerlier, D., 3. Virol.73:5220-4 (1999)), or create new strains replicating more efficiently inhuman cells compared to their wild type isolates. Such viruses may alsoobtain a higher pathogenic potential.

For the above reasons vaccine manufacturers are reluctant to switch tomammalian cell lines and a need for immortal avian cell lines hasdeveloped.

The investigation of tumor induction in birds by the avianalpharetroviruses provided first molecular insights on celltransformation in general. The retroviral oncogenes are derived fromcellular genes with essential regulator domains mutated or deleted. Someof the factors that have been identified in the course of these studies,such as v-myc or v-ras, directly affect components of bothretinoblastoma (RB) and p53 pathways. Other proteins, such as v-src orv-erbB, are constitutively activated (hence, dysregulated) signaltransducers that mimic impinging extracellular mitogens. The problemwith these factors is that they target only one of several pathwaysrequired for efficient transformation. The presence of v-src or v-mycpredisposes the cell for transformation and requires additional,spontaneous and unpredictable alterations within the cell for fulltransformation. The risks for the patient posed by cells transformedwith one of the retroviral oncogenes therefore is difficult to estimate.

In other cases a single strong tumor antigen (e.g. v-jun) is able todirectly cause tumor formation (Hartl, M. et al., Curr. Cancer DrugTargets 3:41-55 (2003)). Many avian viral oncogenes maintain theironcogenic potential in mammalian cells.

Cell lines created by these viruses are not suitable for vaccinemanufacturing. A retrovirus carrying an oncogene may get activated andtransferred together with the vaccine. Even a tumor antigen not enclosedby viral LTRs may pose a high risk when it is able to transformmammalian cells without the help of complementary antigens. This risk istypically estimated by consideration of the transforming potential, thenumber of vaccinees, and the amount of cellular nucleic acid transferredwith the vaccine virus. This amount is limited by the efficiency of thepurification process and currently cannot be reduced to below 10pg/dose. This criterion is especially stringent for vaccine productionwhere a healthy population often is inoculated at a very young age.

The same arguments apply to transforming DNA viruses such aspapillomaviruses and polyomaviruses. These viruses are equiped withaggressive oncogenes: SV40 large T antigen is a multifunctional proteinwhich affects both checkpoint control in G1 of the cell cycle and p53activity. Therefore, large T readily immortalizes and transformsmultiple mammalian tissues of rodent and human origin. With the additionof small T antigen (further enhancing large T action and additionallymodulating the AKT3 pathway) it was possible to immortalize avian cells(part of patent application US 2001-0016348). However, even withsophisticated modern purification methods SV40 large-T antigen isconsidered too aggressive for use in cell lines generated forapplication in human medicine. In contrast to the above, the genesproposed in this invention affect checkpoint control of the cell cycleand p53 inactivation via separate factors: a required simultaneoustransfer event of two distinct factors for transformation dramaticallydecreases any theoretical risk for the vaccinee.

US patent application 2001-0016348 describes the use of ananti-apoptotic pathway completely unrelated to the present invention. Itdoes not provide a second gene that counters an internal signal forapoptosis due to forced cell cycle progression caused by a first gene.Apoptosis can also be induced by a variety of external simuli, forexample lack of growth factors or loss of anchorage. Transmission ofthis type of pro-apoptotic signal can be inhibited by bcl-2 familygenes, the focus of US patent application 2001-0016348.

Whereas 90% of cervix carcinomas carry papillomavirus sequences, C-typeadenoviruses (which include types 2 and 5) are considered not to inducetumors in vivo, and adenoviral sequences have not been detected in humantumor tissue.

Alternatively, it has been tried to develop cell lines by continuouspassaging of chicken embryonic fibroblasts. Whereas rodent cells appearto undergo spontaneous immortalization quite easily (Curatolo et al., InVitro 20:597-601 (1984)), avian and primate cells are highly resistantto this approach (Harvey, et al., Genes and Development 5:2375-2385(1991); Pereira-Smith, 3. Cell Physiol. 144:546-9 (1990); Smith et al.,Science 273:63-67 (1996)). Somatic cells of avian or primate origin lacktelomerase and senescence is caused by the shortening of chromosomalends (telomeres). Nevertheless, a chicken fibroblast line UMNSAH-DF1 hasbeen developed using this approach (U.S. Pat. Nos. 5,672,485 and6,207,415). Immortalization by this approach is caused by spontaneousmutations in multiple oncogenes or tumor suppressor genes. This is arare event which is unlikely to be reproduced especially in cells ofother tissue origin. Most importantly, such an approach contradicts theDefined Risk approach as a general rule for human live vaccinesproposing detailed knowledge about the immortalizing genes to assess therisk of oncogene transfer. Again, according to the FDA (CBER Discussionfrom May 12, 2000, on cell substrates) the use of neoplastic cellsderived from naturally occurring tumors or cells that have beentransformed by unknown mechanisms is discouraged for the development ofminimally-purified live-attenuated viral vaccines and virus-vectoredvaccines.

The spontaneously developed UMNSAH-DF1 chicken fibroblast line exhibitsalterations in E2F and p53 activity (Kim et al., Oncogene 20: 2671-82(2001)). This is not surprising because enhanced cell cycle activityrequires active E2F, and because it is known from mammalian cell studiesthat high E2F activity induces apoptosis in the presence of active p53.The study characterizes the immortal stage without shedding light on thecausative events: mutations in a large number of genes may have causedimmortalization.

A spontaneous transformation process may be enhanced by the use ofchemical mutagens (U.S. Pat. No. 5,989,805). The particular cell linesgenerated using this approach have overcome senescence but maintained afibroblast like appearance and are non-tumorigenic. Although thisrepresents a significant safety feature, these cells are of low valuefor large scale fermentation techniques. Furthermore, this chance-basedapproach also contradicts the Defined Risk guidelines.

Avian cell lines originating from naturally occurring tumors such as aquail fibrosarcoma (WO 97/08307) have also been proposed forbiomanufacturing. Again, the Defined Risk guidelines for use in humanvaccine production are violated by a method that is based on chanceevents.

The approaches taken in the studies described above are in sharpcontrast to the active introduction of specific groups of immortalisinggenes according to this invention, which defines the causative agentsfor immortalization and allows to assess risk, provides high flexibilitywith respect to selection of various tissues, and allows to modulatecertain features of the resulting cell line.

Despite the fact that chicken eggs and fibroblasts have a considerabletrack record they are also associated with a very specific risk factorthat only recently has come into greater focus: chicken cells release atleast two types of retroviral particles, the endogenous avian retrovirus(EAV) and the endogenous avian leukosis virus (ALV-E). The issue issimilar to the presence of endogenous retrovirus particles in mousecells which are used for the manufacture of recombinant proteins (suchas NS0). However, in contrast to mouse cells, chicken cells have beenshown to contain reverse transcriptase. Due to more efficient detectiontechniques RT activity has also been detected in chicken cell-derivedmeasles, mumps and yellow fever vaccines (Hussain, A. I. et al., 3.Virol. 77:1105-11 (2003); Shahabuddin, M. et al., 3. Clin. Microbiol.39:675-84 (2001)). Whether the presence of reverse transcriptaseactivity results in transmissible retroviruses remains controversial: amore detailed analysis has shown that CEF (from White Leghorn) containfive loci with integrated EAVs, two of which can express infectiousALV-E whereas the other three are defective (Johnson, J. A., Heneine,W., 1 Virol. 75:3605-12 (2001)). Tsang, S. X. et al., 3. Virol.73:5843-51 (1999) also found RT activity and release of viral particlesbut did not observe any transmission after a careful search for EAVsequences in blood mononuclear cells of children that received mumpsvaccine. According to the Weekly Epidemiological Record of the WHO (73)28 (1998), independent laboratories have investigated the infectivity ofthe particles for a variety of human and other mammalian cells byextensive co-cultivation and could not detect transmission of RTactivity or productive infection. This finding is supported byepidemiological studies that have revealed no association between theuse of chicken cell-derived vaccines and incidence of cancers, includingthose of childhood.

Furthermore, in the mentioned Weekly Epidemiological Record, the WHOstresses the importance of chicken host cells to maintain attenuation ofcertain vaccine strains. Alternative production processes are notcurrently available, and this lack of alternatives is an importantreason for the acceptance of a known and continous source for a viralcontaminant.

However, epidemiological studies superimpose populations and do notinvestigate chance events or case studies. Epidemiological studiescannot refute theoretical risks, for example: the accepted endogenous RTactivity may mask RT activity from unacceptable exogenous contamination,and the endogenous viruses may be mobilized and activated if packagingconstructs are introduced into the cells (Ronfort, C. et al., Virology207:271-5 (1995)).

It was shown, however, that cells from ducks and geese do not containEAV and ALV related sequence and the Japanese quail is free of reversetranscriptase (Smith, L. M. et al., 3. Gen. Vrol. 80(pt1):261-8 (1999);Brudno, I. A. et al., Vopr. Virusol. 97-100 (1980)).

Adenoviruses (AdV) are well characterized, naked (non-enveloped)ubiquitous viruses. For the most common serotypes Ad2 and Ad5 theseroprevalence in the human population approaches 90%. Replicationincompetent versions of these viruses are used as gene therapy andvaccine vectors in trials with human patients. Genes from the E1 regionof human Adenovirus 5 have been used to transform some specific humancells in vitro (293 and PER.C6 cell lines; Fallaux, F. J. et al., Hum.Gene Ther. 9:1909-17 (1998); Graham, F. L. et al., J. Gen. Virol.36:59-74 (1977)). The general process is inefficient compared tostronger multifunctional oncogenes such as SV40 large T antigen. Basedon the observation that 293 show neuron specific markers and PER.C6 areof neuroectodermal origin it was suggested that Ad5 E1.-basedtransformation is limited to neuronal cells (Shaw et al. Faseb J 16(8):869-71 (2002)). Considering the significant species barrier betweenhuman and avian cells efficient immor-talisation of multiple aviantissues by transfection is even more unexpected.

Mammalian E1 transformed cell lines have been used for the production oflive purified adenovirus vectors in clinical trials. With carefulmonitoring of the amount of contaminating cellular DNA in a vaccinepreparation and its size, the transforming genes of Ad5 are notconsidered a safety hurdle (Vaccines and Related Biological Productsadvisory committee, session from May 16, 2001).

Adenoviruses replicate in the nucleus of the infected cell. Becausequiescent host cells are not permissive for a full viral life cycleadenoviruses have evolved mechanism to force cells into S-phase. Tomaximize burst size of progeny viruses they have also evolved mechanismto evade apoptosis as a response of the host cell to capsid penetrationand viral replication. The genomic region that mediates both cell cycleprogression and inhibition of apoptosis is the E1 region.

The E1 region actually consists of two distinct expression cassettes,E1A and E1B, arranged in tandem and each equipped with its own promoterand polyadenylation site. At least three proteins are translated fromthe E1A primary transcript by alternative splicing. Among others, E1Aproteins have been found to disrupt RB/E2F complexes and to interferewith the p300 and CBP transcriptional co-activators. The escape of E2Fsfrom the RB repressor induces progression of the cell cycle from G1 to Sphase, whereas the E1A/p300 complex induces apoptosis via severalpathways (Putzer, B. M. et al., Cell Death Differ. 7:177-88 (2000)),including repression of transcription of MdM2, a negative regulator ofthe key sensor for apoptosis, p53.

As E1A sensitizes cells to TNF-induced apoptosis it is considered anantitumor agent, and it is used in experimental approaches for tumortreatment (Lee, W. P. et al., Cancer Res. 63:6229-36 (2003)).

Furthermore, acting as a transcription modulator it drives cells towardsde-differentiation, a feature advantageous to a potential cellsubstrate.

It was shown by Guilhot et al. (Guilhot, C. et al., Oncogene 8:619-24(1993)) that retroviral transduction of the 12S protein of E1A from Ad5can lead to immortalization of quail cells. This is likely theconsequence of interaction between the avian RB and EIA. However, theprocess fails when the gene is introduced by transfection of naked DNAinstead of retrovirus infection (pers. observation). We propose that theextremely efficient and stable transduction via retrovirus infectioncreates a cell pool large enough to harbor individual cells withspontaneous genomic changes that have blocked apoptosis that normally isinduced upon RB inactivation. These required but unknown changesincrease the risk for vaccinees and the resulting cell line cannot beconsidered a designer cell line (the result of defined blocks inspecific pathways). Moreover, the transforming gene introduced viaretroviruses is flanked by inverted terminal repeats and can, therefore,be mobilized. Such an event may even be more pronounced in cell linesthat express reverse transcriptase from endogenous retroviruses.

SUMMARY OF THE INVENTION

In view of the above, it is still desirable to develop an avian cellline with convenient growth properties for large scale manufacture,using a defined combination of immortalizing/transforming genes. It isfurther desirable that none of these genes is able to transformmammalian cells independent of the other genes. Moreover, the action ofa single gene should either have no immortalizing/transforming effect orresult in apoptosis of cells expressing the respective gene. The risk ofjoined transfer to a vaccine recipient should further be minimized bypositioning the respective genes on separate expression units. Finally,it would be desirable—as the human population is typically exposed tothe respective genes—that these genes are not associated with tumorformation in the human population. The cell line to be generated shouldnot release infectious virus particles from endogenous retroviruses ornot exhibit reverse transcriptase activity at all.

It was found that transformation of avian cells with two particularviral and/or cellular genes, one of which affecting the retinoblastomaproteins and the other the p53 protein, provided for a cell line wellsuited for the production of viruses for vaccination.

The invention thus provides:

(1) an avian cell line immortalized with a combination of viral and/orcellular genes (hereinafter shortly referred to as “gene(s)”), at (eastone first gene affecting the function of the retinoblastoma protein andat least one second gene affecting the p53 protein or a family memberthereof, wherein preferably the first gene overcomes G1 checkpointcontrol and the second gene prevents apoptosis induced by the firstgene;(2) a method for preparing a cell line as defined in (1) above, whichcomprises transforming/transfecting a starting cell with the first andsecond gene;(3) the use of the cell line as defined in (1) above for the productionof biologicals or viruses, preferably for the preparation of a vaccineor for gene therapy; and(4) a method for producing viruses or biologicals using a cell line asdefined in (1) above.

SHORT DESCRIPTION OF THE FIGURES

FIG. 1: Schematic sections of the expression plasmids used for enhancedimmortalization of primary duck cells (example 2). Polyadenylationsignals are omitted for clarity. The alphanumerics at the left are shortidentifiers for the plasmids. mPGK and hPGK, phosphoglycerate kinasepromoters of mouse and human, resp.; ad5, E1-endogenous promoter of Ad5;moCMV, mouse CMV immediate early promoter; tk, herpes simplex virusthymidine kinase promoter; orf 22 and gam1, CELO virus genes; E1A andE1B, adenovirus 5 E1 region genes.

FIG. 2: Phase contrast microscopy pictures as example of focus formationin Ad5-E1 transfected duck embryonal liver cells (plasmid 49E). A,initial magnification 4× to depict a complete focus embedded insenescent primary cells. B, initial magnification 20×: perimeter of alarge round focus of small cells arranged in a compact monolayer visibleat the right of the panel, primary cells in advanced senescence towardsthe left.

FIG. 3: Immunofluorescence assay for E1A and E1B 55K proteins (example3). Upper two rows, mix of plasmid 49E-immortalized and primary duckliver cells; bottom two rows, 293 positive control cells. Left column,phase contrast images; middle column, immunostaining of E1A or E1B 55Kproteins as indicated in the images; right column, DAPI stain. The E1B55K protein characteristically localizes to the cytoplasm andaccumulates in aggregates to yield an uneven, spotty distribution. E1Ais a nuclear protein. Note the compacted nuclei that stain brightly withDAPI in the transformed duck cells.

FIG. 4: Q-PERT assay (quantitative PERTassay) on cell supernatant fordetection of retroviral activity (example 4). Bold squares, CHO positivecontrol; open squares, water negative control; bold diamonds, chickenembryonic fibroblasts; bold triangles, 293 cell line negative control;grey circles, substrate-only negative control; open triangles, duckliver cells immortalized with plasmid 49E; delta Rn, emission of thereporter dye over starting background fluorescence.

FIG. 5: MVA amplification on some of the described duck cell lines andCEFp (example 5). Infection was performed with an MOI of 0.1. Titrationwas performed on VERO cells 48 hours after infection (Example 2). CEFp,primary chicken embryonic fibroblasts.

FIG. 6: serial passaging of MVA on duck retina cells immortalized withplasmid 49E (example 5). Bold squares, burst size; bars, input virusadjusted to an MOI of 0.1. Input virus is given as reference todemonstrate that burst size is independent of experimental fluctuationsin cell numbers (which in turn define input virus via MOI).

SEQUENCE LISTING Free Text

SEQ ID NO: Description - free text 1 Primer VS182 2 Primer VS183 3Primer VS184 4 Primer VS185 5 Primer VintSA-F 6 Primer VintSA-R 7Plasmid pEFAd5E1A 8 Plasmid pEFAd5E1BSA 9 Plasmid 49E 10 Plasmid 25F 11Primer V206 12 Primer V207 13 Primer V208 14 Primer V209 15 RT primer 16Primer cDNA 1 17 Primer cDNA 2 18 Plasmid 60E 19 Plasmid 36E

DETAILED DESCRIPTION OF THE INVENTION

“Immortalized”, “immortalized cells” and “immortalized cell line”according to the present invention relates to a cell or cell line whichhas been transfected/transformed by certain functional DNA sequencesconferring the potential for at least 200 passages, preferably unlimitednumber of passages, i.e. immortality, to the respective starting cells.

A “gene cassette” of the present invention is to be understood as a DNAsequence comprising a gene affecting the function of the retinoblasomaprotein, i.e. which directly or indirectly (e.g. after expression)mediates the disruption of complexes between retinoblastoma proteins andE2F transcription factors, and which in addition comprises a viral genepreventing induction of growth arrest and apoptosis by p53 such as theadenovirus E1B 55K protein of all groups, the E6 protein ofpapillomaviruses, preferably those of the low-risk humanpapillomaviruses (HPV) (such as HPV1, HPV6 and HPV11, but not HPV16,HPV18), or a cellular gene preventing growth arrest and apoptosis by p53such as mdm2.

In more detail, the above gene cassette comprises a “first gene” whichin a preferred aspect of (1) directly or indirectly (e.g. via cellularinducers) mediates the disruption of complexes between retinoblastomaproteins and E2F transcription factors. This first gene may be a viralgene such as a mastadenovirus E1A, gam1 and orf22 of CELO or E7 ofpapillomaviruses, preferably of the low-risk human papillomaviruses(such as HPV1, HPV6 and HPV11, but not HPV16, HPV18), or a cellular genesuch as a constitutively active CDK4 or an over-expressed D typecycline. The activity of the first gene mediates cell cycle progressionusually at the cost of induction of apoptosis or growth arrest withincreased passaging.

A “second gene” is present in above gene cassette to counter this effectof the first gene. It prevents apoptosis or growth arrest and preferablyacts by inhibiting transcriptional activation by p53 via augmenting thedegradation of p53 or converting p53 from a trans-activator to arepressor of transcription. Preferably the “second gene” is capable ofpreventing transcriptional activation by p53, including repression ofthe function of p53 and causing a decrease in stability of p53. The“second gene” may be a viral gene such as the adenovirus E1B 55K proteinof all groups, orf22 of CELO, the E6 protein of papillomaviruses,preferably of the low-risk human papillomaviruses (such as HPV1, HPV6and HPV11, but not HPV16, HPV18), or a cellular gene preventing growtharrest and apoptosis by p53 such as mdm2. Preferably the “second gene”is orf22 of CELO or adenovirus E1B 55k.

This is exactly opposite to the introduction of exogenous active wildtype p53 which was associated with the generation of a chickenfibroblast line by an unknown mechanism (U.S. Pat. No. 5,879,924).

“Biologicals” in the context of present invention comprises therapeuticand recombinant proteins, including antibodies, enzymes, hormones,receptors or their ligands and fusions thereof. Prefererred biologicalsare recombinant proteins.

One preferred aspect of embodiment (1) is the use of a cell line derivedfrom embryonic or hatched chicken, duck, goose, quail or the like,preferably from chicken or duck. In an especially preferred aspect of(1), additionally this cell line is free of reverse transcriptaseactivity, derived from immortalization of a primary cell originatingfrom chicken embryos, hatched chicken, duck embryos or hatched ducks, isderived from extraembryonic membrane and/or is cultivated in achemically defined medium. The medium is preferably free of animalserum.

Another preferred aspect of embodiment (1) is that the cells subjectedto immortalization are primary cells including fibroblasts, cells fromisolated body segments (somites) or separated individual organsincluding neuronal, brain, retina, kidney, liver, heart, muscle andextraembryonic tissues and membranes protecting the embryo. Mostpreferably, the cells are from extraembyonic membranes or retina.

The immortalization leading to the cells of embodiment (1) is preferablyeffected by non-viral transfection, including, but not limited to,transfection mediated by liposomes, dendrimers or hydroxyapatite(“calcium phosphate”) precipitates and electroporation.

Preferably, the first gene in embodiment (1) is a viral gene mediatingdisruption of complexes between retinoblastoma proteins and E2Ftranscription factors. This includes, but is not limited to, anadenovirus E1A gene from mastadenoviruses (preferably frommastadenoviruses of group C), an E7 protein of papillomaviruses,preferably from low-risk human papilloma virus (HPV) (such as HPV1, HPV6and HPV11, but not HPV16, HPV18), an orf 22 gene of avian adenovirusesand/or E43 open reading frames from ovine attadenovirus. Alternatively,the first gene of embodiment (1) is a cellular gene mediating disruptionof complexes between retinoblastoma proteins and E2F transcriptionfactors. This includes, but is not limited to, cyclin D1, cyclin D2,cyclin D3 and/or a mutated CDK4 not susceptible to inactivation byp16INK4a.

The second gene of embodiment (1) is preferably a viral gene coding fora protein preventing induction of growth arrest and apoptosis by p53.This includes, but is not limited to, genes coding for the adenovirusE1B55K protein of all groups, GAM-1 of CELO, the E6 protein ofpapillomaviruses, preferably those of the low-risk HPV (such as HPV1,HPV6 and HPV11, but not HPV16, HPV18). Most preferred are genes codingfor the adenovirus E1B55K protein and GAM-1 of CELO. Alternatively, thesecond gene encodes a cellular protein preventing growth arrest andapoptosis by p53 such as mdm2.

The first gene and second gene of embodiment (1) are preferably eitherseparated spatially by heterologous sequences or located on differentnucleic acid segments or plasmids.

In an especially preferred aspect of embodiment (1) the first gene isthe E1A and the second gene is the E1B region of an adenovirus from thegenus Mastadenovirus, preferably from adenovirus 5. Most preferably saidE1A regions have the sequence of by 1193 to 2309, preferably by 1239 to2309, of SEQ ID NO:7 or the sequence complementary to by 4230 to 3113 ofSEQ ID NO:9. Furthermore most preferably said E1B regions have thesequence of by 1145 to 3007, preferably by 1197 to 2810, of SEQ ID NO:8or the sequence complementary to by 2345 to 550 of SEQ ID NO:9.

In a further especially preferred aspect of embodiment (1) the firstgene is orf22 and the second gene is GAM-1 from an adenovirus,preferably from the genus aviadenovirus CELO, which preferably have thesequence represented by the sequence complementary to by 1252 to 635 ofSEQ ID NO:10, and the sequence complementary to by 3138 to 2290 of SEQID NO:10.

In even a further especially preferred aspect of embodiment (1) and (2)the plasmids 36E (SEQ ID NO:19), 37E (FIG. 1), 49E (SEQ ID NO:9), 25F(SEQ ID NO:10) or 60E (SEQ ID NO:18) are used for immortalization of thecells.

Furthermore, combinations of nucleic acids encoding E1A and/or E1B withGAM-1 and/or Orf22 as defined above are preferred aspects of embodiment(1).

The cell line according to embodiment (1) may additionally carrynon-natural functional sequences including, but not limited to,transgenes such as genes complementing deficient viruses (e.g. EBNA1,etc.), promoters (e.g. PGK-, EF1.alpha-, CMV-promoter, E1-promoters ofAd5, tk-promoter etc.), enhancers (e.g. RSV-LTR), selection markers suchas neomycin-resistance, puromycin-resistance, etc. In one preferredaspect the first and second gene are under the control of separatepromoters selected independently from PGK-, CMV-, E1- and tk-promoters.

The cell line according to embodiment (1) is in one preferred aspectfurther-more suitable for production of biologicals or viruses includingvaccine strains (Marek's disease, infectious bursal disease, Newcastledisease, turkey herpes, chicken anemia, influenza, vaccinia (MVA),rubella, rabies viruses, etc.) and recombinant viral vectors (e.g.recombinant MVA or alphaviruses). Most preferred viruses for vaccinationare MVA and influenza viruses. The most preferred recombinant viralvector is MVA.

In one aspect of embodiment (1) the cell line is cell line 12A07-A10(DSM ACC2695) derived from immortalization of duck extraembryonalmembrane cells with plasmid 49E (example 2).

Furthermore preferred is the generation of the cell lines according toembodiment (1) under cGMP conditions which renders them suitable forpharmaceutical application.

The method of embodiment (2) preferably comprises non-viral transfectionof the starting cell such as listed above. Most preferred is liposomaltransfection, especially transfection by the Effectene reagent.

A preferred use according to embodiment (3) is the use for thepreparation of a vaccine or for gene therapy. A viral vaccine strain orgene therapy vector is brought into contact with cells of a cell lineaccording to embodiment (1) so that infection occurs and the virus isamplified by said cells. Continued passaging of virus (repeated cyclesof infection and harvest of virus on said cells) will lead toattenuation or adaptation of virus to this particular host cell line.Thus, a viral vector or vaccine strain with lesser virulence for theintended vaccinee (which is not duck, preferably not avian) isgenerated. Attenuated viruses allow the immune system of the vaccinee tolaunch a response that is more protective than vaccination with fullyinactivated particles, and that is less severe than infection with awildtype (natural) pathogen. The preferred viruses for this embodimentare measles and rabies viruses.

The method for producing viruses according to embodiment (4) preferablycomprises the contacting of said viruses with a cell line according toembodi-ment (1) and/or the cultivation of said viruses on said cellline. Especially, this method can be used for producing a pox virus,preferably strain MVA, in a duck cell line, preferably a cell lineoriginating from duck somites or duck neuronal tissue, even morepreferred from duck retina. Especially duck retina and somite-derivedcells obtained by transfection of Ad5-E1 region under cGMP conditionsstably support amplification of MVA with an efficiency comparable to orbetter than primary chicken embryonic fibroblasts (Example 5).

The method for producing biologicals, especially recombinant proteins,according to embodiment (4) comprises the introduction of a gene codingfor a recombinant protein, operably linked to a promoter into a cellline according to embodiment (1), cultivating said modified cell lineand harvesting the recombinant protein.

The method of embodiment (4) is used preferably for the production ofviruses and biologicals usable for vaccination or gene therapy.

Historically, chicken eggs and the respective cells (chickenfibroblasts) are the dominating substrate for the manufacturing ofvaccines. For pharmaceutical purposes chicken are available frompathogen-controlled environments with an extensive monitoring system. Alarge body of literature suggests chicken eggs as the primary target forcell line development. Therefore, chicken cells are one preferred sourcefor starting cells of the invention. However, chicken-derived cells andcell lines will be most likely RT positive. Literature data suggest alow risk for release of infectious virus. However, the absence oftransmissible virus will have to be monitored for any cell line to beused in manufacturing. Indeed, most of the avian cell lines establishedso far are originating from chicken (U.S. Pat. No. 5,830,723, U.S. Pat.No. 5,879,924). Although it was possible to breed a chicken lineage(line 0) free of avian leucosis virus, endogenous avian retroviruses(EAV-HP) (Boyce-Jacino et al., J. Virol 66(8):4919-29 (1992)) arepresent in chicken cells including line 0. EAVs provide an activereverse transcriptase, but expression levels vary substantially.Therefore, even primary chicken cells and cell lines such as DF1 thattested RT negative in less sensitive assays (Crittenden et al., Virology57(1):128-38 (1974)) presumably will test positive in modern real timePCR approaches and may harbor retroviruses that are activated undercertain growth conditions.

Alternatively preferred avian species of this invention for cell linedevelopment are those which do not contain endogenous retroviruses orexpress reverse transcriptase (RT). This includes ducks, which aresuitable for two additional reasons: Duck eggs are also available frompathogen free monitored stocks and ducks are, in contrast to geese, lesslikely to develop spontaneous tumors. While it is known that many of therelevant vaccine strains replicate well in duck (embryonal) cells asthey do in chicken (embryonal) cells (e.g. Marek's disease virus(Witter, R. L., Avian Dis. 46:925-37 (2002)) or rubella (Rocchi, G.,Salvador', A., Nuovi Ann. Ig Microbiol. 21:336-40 (1970))), this remainsto be shown for virus strains of primary interest. For other vaccinessuch data is not available.

To our knowledge it is a novel and unexpected finding of this inventionthat the highly attenuated pox virus strain MVA (modified vacciniaAnkara) replicates in duck cell lines at similar or higher efficienciesthan in commonly used primary chicken embryonic fibroblasts. Oneintention of the inventors was to provide a safe and robust alternativeto primary cells for amplification of viruses that require an avianhost, or vaccine strains where a non-mammalian host is preferred. Animportant virus for which convenient host cells are not available is MVA(modified vaccinia virus Ankara). MVA is a highly attenuated pox virusand an extremely promising tool for therapeutic and protective vaccineapplications. MVA will serve as a model virus for characterization ofduck cells but should not be taken as an exclusive example: thedescribed experiments can also be performed with a range of otherviruses, whether pathogens or therapeutic vectors, such as measles,rubella, rabies, or influenza viruses.

Fibroblasts have been selected as the preferred cell type mainly forhistoric and practical reasons. Fibroblasts are the fastest growingprimary cells from mammalian as well as avian species. When a cellsuspension from whole chicken embryos is brought into culture, this isnot the only but the predominant cell type. However, fibroblasts growstrongly adherent and loose this feature only after complete(tumorigenic) transformation. This process requires the presence ofstrong transforming genes such as v-ras interfering with signaltransduction pathways. Early senescence of fibroblast cultures is inpart caused by the total absence of telomerase activity in birds and man(Forsyth, N. R. et al., Differentiation 69 (4-5):188-97 (2002)).

Human primary fibroblasts are refractory to transformation with the E1genes of adenovirus type 5 which do not directly interfere with thesepathways (personal observation). Efficient immortalization and growth insuspension culture has a higher chance to succeed for epithelial andneuronal cells. Moreover, epithelia instead of fibroblasts seem to bethe primary site for virus replication inside the bird egg.Interestingly, in contrast to the human situation, bird kidney doesexpress telomerase throughout life which makes bird kidney cells a goodtarget for immortalization. Taken together, bird epithelial cellsincluding kidney epithelium and neuronal cells are considered the mostpromising targets to develop a cell line of the required features.

It is therefore only for the ease with which fibroblasts are obtainedthat avian cell line development has almost exclusively focused on thesecells (Cowen, B. S., Braune, M. O., Avian Dis 32(2):282-97 (1988); U.S.Pat. No. 5,830,723). In some cases whole embryos have been used (US2001-0016348).

Viruses do not only exhibit species but also organ and tissuespecificity based on receptor distribution and cellular factorssupporting replication. Therefore, in contrast to the typical approach,a preferred way to perform present invention is the separation of organsprior to cultivation to obtain a most preferred host cell.

For influenza virus, whose vaccine-adequate production is a majorapplication for the cell lines of present invention, the typical site ofreplication is not the embryo itself but extraembyonic membranes.Therefore, a specific aim was to also develop cell lines fromextraembryonic material, including protective membranes of the embryo.Some tissue specific primary cultures including those of theextraembryonic membranes have very short survival times compared tofibroblasts. This further highlights the need for designedimmortalization to obtain optimized host cells. Successfulimmortalization of multiple tissues in a limited time window requiresthe specific combination of genes used within present invention.

It was not known which of the avian tissues has the highest replicativepotential for pox viruses such as MVA or Canarypox. The typicalmanufacturing process for MVA involves a mixture of cells from an embryoexcluding the head which is removed prior to disintegration. It istherefore completely unexpected that a cell line of neuronal origin,developed from the retina, has such a high capacitiy for MVA replicationwhereas other tissues have not.

The same tissue specificity applies to protein production. Thetranscriptional capacity is dependent on the available set oftranscription factors and even strong ubiquitous viral and cellularpromoters exhibit variable strength in different tissues. Moreover,yields of secreted protein strongly depend on the capability of aparticular cell type to fold and process (e.g. glycosylate) the proteinproperly.

The mechanisms leading to immortalization and transformation of primarycells have been well described (Hahn, W. C. et al., Nature 400:464-8(1999)). Required elements interfere with (1) control of cell cycleprogression, (2) programmed cell death induced by the deregulated cellcycle, (3) growth factor signal transduction and for human and aviancells (4) shortening of the telomeres, the linear termini of thechromosomes. A large number of factors are known that can drive primarycells to an immortalized and transformed phenotype but immortalizationcomes at the cost of inhibiting cellular checkpoints that areresponsible to minimize tumor formation in the host. It is thereforedesired to select transforming factors that can effect experimentalgeneration of a cell line but pose a minimal risk of tumor induction inthe recipients of biologicals derived from the designer cells. Thisrequirement needs to be balanced with the strength of the transformingfactors: they should be strong enough to cause transformation withoutthe need for accumulation of additional spontaneous mutations; that is,the molecular pathway leading to the resulting cell line should be knowncompletely (categories I and II according to the FDA CBER Office ofVaccine's presentations at the May 2000 Advisory Committee). It isfurthermore desired to select a synergistic combination of factors thatindividually cannot transform primary cells so that a concurrenttransfer of genetic material is required which further minimizes therisk of inadvertent transformation in vaccinees or patients. Finally, itis desired that the transforming factor elicits an immune response inthe recipient of biologicals so that immune tumor surveillance isactivated in the unlikely event of tumor formation due to productapplication. The last criterion can be realized if non-cellular butforeign, for example viral, transforming proteins are utilized.

It was now found that the E1 region from human adenovirus 5 (Ad5) isideally suited to transform avian cells so that the resulting designercell complies with all of the above criteria.

The E1B region encodes two open reading frames on a bicistronic mRNA,the 21K and 55K proteins. The 55K protein binds to p53 and thus turnsthe pro-apoptotic transcriptional activator into a repressor. The 21Kprotein comple-ments this anti-apoptotic activity by binding to Bax,thus maintaining integrity of the mitochondrial membrane and preventingthe release of cytochrome C. This protein is essential to drive adherentcells towards substrate independent growth and hence is essential to afermentation process in suspension.

It has not been shown before whether human adenovirus E1B 55K can affectthe avian homologues of p53. Furthermore, the avian adenoviruses are notequipped with genes resembling E1B so that inference also was notpossible. Contrary to all expectations, the inventors have found thatE1B can provide the essential functions to allow immortalization by E1A.

A novel and crucial factor for the here described achievement wasremoval of E1B from its weak natural context and placement under controlof a strong, recombinant promoter. This novel modification andcombination allowed efficient immortalization of multiple tissues fromduck and chicken by transfection instead of retroviral transduction.

Although the underlying mechanism for transformation by E1 is complexone hallmark is a most desirable feature: E1A is a strong inducer ofcell proliferation and apoptosis whereas E1B proteins efficientlyinterfere with apoptosis but cannot release restriction on cell cyclecontrol.

Hence, not a single factor but the continuous presence of E1A and E1Bproteins are required to sustain the experimentally induced transformedphenotype.

Since the description of v-src in the 1970s (Brugge, J. S., Erikson, R.L., Nature 269:346-8 (1977)) a panoply of transforming factors have beendiscovered and characterized. Indeed, it was the study of induction oftumors in birds by alpharetroviruses that provided first molecularinsights (Martin, G. S., Nature 227:1021-3 (1970)). The retroviraloncogenes are derived from cellular genes with essential regulatordomains mutated or deleted. Some of the factors that have beenidentified in the course of these studies, such as v-myc or v-ras,directly affect components of the RB and p53 pathways. Other proteins,such as v-src or v-erbB, are constitutively activated (hence,dysregulated) signal transducers that mimic impinging extracellularmitogens. The problem with these factors is that they target only one ofseveral pathways required for efficient transformation. The presence ofv-src or v-myc predisposes the cell for transformation and requiresadditional, spontaneous and unpredictable alterations within the cellfor full transformation. The risks for the patient posed by cellstransformed with one of the retroviral oncogenes therefore is difficultto estimate.

Other DNA viruses such as papillomaviruses and polyomaviruses are alsoknown to transform cells in vitro. However, the selected transgenesshould not be too aggressive to minimize the risk of tumor induction inthe recipients of biologicals via inadvertently transferred cellularDNA. This criterion is especially stringent for vaccine production wherea healthy population often is inoculated at a very young age. Even withsophisticated modern purification methods polyomavirus Large-T antigenis considered too aggressive for use in cell lines generated forapplication in human medicine. Whereas 90% of cervix carcinomas carrypapillomavirus sequences (Munoz, N. et al., N. Engl., J. Med.34816):518-27 (2003)) C-type adenoviruses (which include type 2 and type5) are not considered to induce tumors in vivo and adenoviral have notbeen detected in human tumor tissue.

Based on the complementary features of the transforming genes shownabove, it was found that a combination of genes each interfering withsingle pathways in the cell cycle and apoptosis is necessary to obtain agenetically stable cell line growing in suspension.

It was shown that the complete E1 region of adenovirus 5 can fulfillthese requirements. Whereas it was shown, that the 12S protein of E1Afrom Ad5 can interact with avian RB (Guilhot, C. et al., Oncogene8:619-24 (1993)) the functional activity of 55K and 21K proteins inavian cells is demonstrated for the first time in present invention. Itis not surprising that some clones of quail cells expressing the 12Sprotein of E1A exhibit transformed features (Guilhot, C. et al.,Oncogene 8:619-24 (1993)). The extremely efficient and stabletransduction via retrovirus infection creates a large enough cell poolto allow individual cells to overcome the cell cycle block or inductionof apoptosis by spontaneous genomic changes. These required but unknownchanges increase the medicinal risk and the resulting cell line can notbe considered a designer cell line, which should be based on knowngenes. Moreover, transfection techniques are not sufficient to createthe large clone pool required for natural selection. Instead retrovirustransduction was required. The transforming gene introduced via thisapproach will be flanked by ITRs and can, therefore, be mobilized, evenmore in a cell line expressing reverse transcriptase.

Recently, an avian adenovirus, termed fowl adenovirus type 1 strain CELO(for chick embryo lethal orphan), has been described in greater detail(Chiocca, S. et al., J. Virol. 70:2939-49 (1996)). Large, centralgenomic stretches of CELO are homologous to Ad5 but differ in importantaspects—among others, CELO is not equipped with an E1-homologous region.Furthermore, CELO cannot complement Ad5 mutagenized in E1A and,conversely, Ad5 E1 proteins cannot trans-activate transcription ofdelayed-early CELO genes (Li, P. et al., J. Gen. Virol. 65(Pt10):1817-25 (1984)). And yet, CELO is capable to transform hamster cellsin vitro (May, J. T. et al., Virology 68:483-9 (1975)). Genesinterfering with cell cycle and apoptosis, orf22 and GAM-1, have beenidentified in the CELO virus (Lehrmann, H., Cotton, M., J. Virol.73:6517-25 (1999)). orf22 encodes a protein that interact with RB, andGAM-1 interferes with apoptosis in a fashion similar to the prototypical21K protein (Chiocca, S. et al., J. Virol. 71:3168-77 (1997)).

It was now found that the genes orf22 and GAM-1 from CELO virus aresuitable substitutes for E1A and E1B. The spectrum of availabletransgenes for transformation of avian cells is therewith expanded.These proteins have not been used previously to transform avian cells.

Furthermore, one of the viral genes may be replaced by a cellular gene.Candidates for such replacement are E2F family members or D groupcyclins for the E1A region of adenovirus and mdm2 for the E1B region.

The following cell lines were deposited at the DMSZ, Deutsche Sammlungvon Mikroorganismen and Zellkulturen GmbH, Mascheroder Weg 1b, 38124Braunschweig, Germany:

1. PBG04 as DSM ACC2577, deposited on Sep. 18, 2002;2. 12A07-A10 as DSM ACC2695, deposited on Oct. 20, 2004.

The invention will be explained in more detail by reference to thefollowing Examples, which are, however, not to be construed as to limitthe invention.

EXAMPLES Example 1 Immortalization of Primary Duck Cells with Adenovirus5 E1A,B

The adenovirus sequences for E1A and E1B were amplified from the cultureof passage 8 of the first generation (E1 deleted) adenovirus Admuc grownin HEK 293 which was heavily contaminated with wild type virus usingprovestart polymerase (Qiagen).

The following primers were used:

(SEQ ID NO: 1) VS182 ACTCGAGCTGACGTGTAGTGTATT (SEQ ID NO: 2)VS183 CACACGCAATCACAGGTTto amplify the E1 A region and

(SEQ ID NO: 3) VS184 ACTCGAGTCATGGAGGCTTGGGAGT (SEQ ID NO: 4)VS185 ACACATTTCAGTACCTCAto amplify the E1 B region. Both fragments were first cloned intopPCR4blunttopo (Invitrogene).

The E1B construct misses the splice acceptor from the E1B message. Itwas therefore replaced by a synthetic one amplified using primers fromthe leader intron of a human immunoglobulin heavy chain. As template,the genomic DNA from PBG04 (DMSZ ACC2577), a murine-humanheterohybridoma was used.

Primers:

(SEQ ID NO: 5) VintSA-F AAGGTACCCTCCCTAGTCCCAGTGA (SEQ ID NO: 6)VintSA-R CAATGTACAGAGTG GGCTCCTGTGG

This splice acceptor was directly cloned into pEFmyc, containing aEF1alpha promoter and the myc leader peptide to create fusion proteins. TheE1A region was removed from ptopoE1A using EcoR I and Xho I sites andcloned into pEFmyc directly, removing the myc leader sequence and fusingthe E1A to the bovine growth hormone poly A. The E1B region was againremoved with EcoR I and Xho I restriction enzymes and cloned intopEFmycSA containing the heterologous splice acceptor site. The resultingplasmids were named pEFAd5E1A (SEQ ID NO:7) and pEFAd5E1BSA (SEQ IDNO:8).

Embryonated duck eggs were incubated at 37° C., 60% air humidity, for 12days (older embryos yielded more cells but also contained a highernumber of contaminating, differentiated fibroblasts). The shell wassterilized with 70% isopropanol, opened at the large end, and the embryowas removed aseptically to a sterile petri dish. The fetal brain andkidneys were removed, transferred to separate petri dishes filled withtrypsin/EDTA and minced. After a brief incubation a suspension thereofwas mixed with an excess of F12 medium (Gibco/Invitrogen) supplementedwith 10% fetal calf serum (Biochrom) and 2% Ultroser G (Ciphergen). Thissuspension was transferred into a petri dish and cultivation wasperformed at 37° C. (which is lower than the 41.6° C. physiologicaltemperature of chicken) and 5% CO₂. The culture medium with non-adherentdebris was replaced the following day and cultivation continued until atleast 5×10⁵ cells per 3.5 cm dishes were available for transfection ofplasmids pEFAd5E1A and pEFAd5E1BSA.

Initial experiments comparing liposomal (Effectene; Qiagen) anddendromeric (Polyfect; Qiagen) transfection reagents suggested bestefficiencies with Effectene. Transfection there was performed using theEffectene reagent; briefly: 2 μg of plasmid DNA was diluted in 200 μl ECBuffer containing 16 μl Enhancer. After an incubation time of 5 min, 16μl Effectene was added. After an incubation time of 10 min, supernatantwas removed from the culture in 3.5 cm dishes and replaced with 1 mlfresh medium containing the transfection mix. After an incubation timeof 2 hours at 37° C. and 5% CO₂, additional 2.5 ml fresh medium wasadded to the culture.

The transfected cells were allowed to reach confluency, trypsinated,resuspended in FCS/Ultroser G-supplemented F12 medium, and re-seededinto two 6 well plates (corresponding to a 12-fold expansion). After 5and 10 days, the medium was replaced with F12 supplemented only with 5%FCS. The plates were scanned for the appearance of foci of cells withchanged morphology (decrease in overall cell size, increased size ofnucleus, increased visibility of plasma membranes under phase contrast)and increased confluency.

Approximately 14 days post transfection, once the foci reached adiameter of 1-3 mm the medium was aspirated and the culture washed twicewith trypsin/EDTA (Gibco). Trypsin-soaked cloning disks (Sigma) wereplaced on top of the aspirated foci for 3 min, then transferred intowells of a 24-well plate filled with 500 μl of F12 medium supplementedwith 5% FCS.

The cloned, transformed cells were allowed to proliferate untilconfluency, trypsinized, resuspended in F12 medium supplemented with 5%FCS and transferred into 6-well plates. Once the culture reachedconfluency in the 6-well plate the cells were transferred to T25 flasksfor continuous passaging.

For cryopreservation at defined intervals cells were trypsinized,resuspended in F12 medium containing 5% FCS, collected by centrifugationat 100 g for 10 min, resuspended in F12 medium containing 50% FCS and10% DMSO (Sigma) to a concentration of approximately 3×10⁶ cells per ml,and placed in cryovials in an isopropanol-based cooling device at −75°C. The cooling device ensures a constant cooling rate of 1° C. per min.After 24 hours the cells were transferred to liquid nitrogen forpermanent storage.

Example 2 Improved Preparation of Immortalized Avian Cell Lines a)Preparation of Primary Cells

The flock of origin for the duck eggs was certified to be free ofSalmonella enteritidis and S. typhimurium; Mycoplasma gallisepticum andM. synoviae; cases of leucosis, reticulo-endotheliosis, psittacosis,avian influenza, duck hepatitis, and Derzsy's disease. The animalsintentionally were not vaccinated against parvovirus and no cases ofparvovirosis were detected. Animals in the flock of origin have beenvaccinated against S. enteritidis and S. typhimurium; Pasteurellamulticodica; the metapneumovirus Turkey rhinotracheitis; and theparamyxovirus causing Newcastle disease.

The eggs were allowed to equilibrate without agitation at roomtemperature and after two days were incubated at 38° C. in a dampchamber, rotated frequently by alternating +45° and −45°.

Duck embryos were sacrificed for isolation of primary cells after one orthree weeks of incubation. Eggs were transferred to a cGMP unit (aclosed laboratory performing as outlined by the Current GoodManufacturing Practices) and the shell was sterilized by wiping with 70%isopropanol under a laminar flow hood. All subsequent steps wereperformed in the GMP unit under sterile conditions with definedsolutions or media.

Eggs were opened carefully, embryos transfered to a large petri dish andkilled immediately by decapitation. Samples from the following organswere removed: brain, retina, liver, esophagus, heart, andextra-embryonic membranes.

In addition, cells from somites were prepared from an 8-day-old embryo.

All samples were rinsed with PBS (phosphate buffered saline;Gibco/Invitrogen, USA), treated with trypsin (Gibco/Invitrogen, USA) for1 to 10 min, and triturated in DMEM:F12 culture medium(Gibco/Invitrogen, USA) supplemented with 10% FCS (Biochrom AG, Germany)by repeated passaging through an 18G syringe. The homogenized sampleswere cultivated at 37° C. and 5% CO₂. Debris was removed from adherentcells by change of medium the following day.

b) Plasmid Constructions

Expression plasmids for E1A, E1B, Orf22, and Gam1 were constructed byextraction of the relevant target regions from the genomic DNA ofadenovirus serotype 5 or chicken embryo lethal orphan (CELO) wildtypevirus, respectively, by PCR and insertion into vectors equipped withhuman or mouse phosphoglycerate kinase (hPGK or mPGK), mouse CMV (moCMV)or tk promoters (FIG. 1).

The adenovirus sequences for E1A and E1B were amplified from wild typevirus using ProofStart polymerase (Qiagen, Germany). The followingprimers were used:

(SEQ ID NO: 1) VS182 ACTCGAGCTGACGTGTAGTGTATT (SEQ ID NO: 2)VS183 CACACGCAATCACAGGTTto amplify the E1 A region and

(SEQ ID NO: 3) VS184 ACTCGAGTCATGGAGGCTTGGGAGT (SEQ ID NO: 4)VS185 ACACATTTCAGTACCTCAto amplify the E1 B region. Both fragments were first cloned intopPCR4-Blunt-TOPO (Invitrogene, USA).

The E1B construct misses the splice acceptor from the E1B message. Itwas therefore replaced by a synthetic one amplified using primers fromthe leader intron of a human immunoglobulin heavy chain. As template,the genomic DNA from PBG04 (DMSZ ACC2577), a murine-humanhetero-hybridoma was used.

Primers Used for Amplification:

(SEQ ID NO: 5) VintSA-F AAGGTACCCTCCCTAGTCCCAGTGA (SEQ ID NO: 6)VintSA-R CAATGTACAGAGTGGGCTCCTGTGG

The genes GAM-1 and ORF-22 were amplified from wild type CELO virus withprimers

(SEQ ID NO: 11) V206 AAC CTC GAG ACC CCC CTG TAC ATT CTA and(SEQ ID NO: 12) V207 GCC GTT AAC TTC AGG GAT TGG TTA CAG, and(SEQ ID NO: 13) V208 CAC CTC GAG TCC GGA TTA AGA TGA ACG and(SEQ ID NO: 14) V209 CCA GTT AAC AGG TGA ACC ATT TAT ACA G,respectively.

Representative examples for the resulting plasmids are given withplasmid 49E (adenoviral factors under control of human PGK and mouse CMVpromoters; SEQ ID NO:9), plasmid 25F (CELO factors under control ofmouse and human PGK promoters; SEQ ID NO:10), plasmid 60E (adenoviralfactors under control of human PGK and tk promoters; SEQ ID NO:18) andplasmid 36E (CELO factor under control of mouse PGK promoter; SEQ IDNO:19) (see also FIG. 1).

Integrity of the expression plasmids was confirmed by sequencing. Theplasmids are not equipped to express resistance factors againstantibiotics (such as ampicillin) in eukaryotic cells.

c) Transfection

Primary cultures were transfected with expression plasmids for E1 orOrf22/Gam1 shortly after isolation or after single subcultivation.Depending on the experiment, plasmids were transfected as supercoils orafter linearization with the Sca I (New Englands Biolabs, USA)restriction enzyme. Initial experiments comparing liposomal (Effectene;Qiagen, Germany) and dendromeric (Polyfect; Qiagen, Germany)transfection reagents suggested best efficiencies with Effectene.Transfection was performed as follows: 2 μg total DNA was diluted into200 μl provided EC buffer and mixed with 16 μl provided enhancer. Afteran incubation for 2-5 min at room temperature 20 μl Effectene reagentwas added. After 5-10 min at room temperature this mixture was appliedto the cells in a 8 cm² dish under 1 ml culture medium. After 2-5 hoursan additional 1.5 ml culture medium was added. On the following day, themedium was replaced with 2 ml fresh culture medium, and thereafter onceper week. Successful transfection was confirmed in parallel experimentswith a reporter gene.

The cells were continously passaged in DMEM:F12 medium containing 10%FCS.

Twenty days after transfection changes of morphology in definedsubpopulations (foci; FIG. 2) of some cultures were observed; in othercultures foci did not appear or were not able to compete with robustproliferation of the primary cells; again other cultures sufferedmassive cell death and senescence shortly after transfection.

A large number of independent foci were expanded from plasmid49E-transfected cultures with cells derived from liver, retina andextra-embryonic membrane. At passage 10, e.g., cell line 12A07-A10derived from duck extraembryonal membrane cells transformed with plasmid49E was isolated and deposited at the DSMZ.

Foci were also obtained from plasmid 60E-transfected cultures with cellsfrom retina and somites.

In plasmid 49E, PGK and mouse CMV promoters drive expression of E1A andE1B, respectively. Plasmid 60E (SEQ ID NO:18) also encodes the fullAd5-E1 region but expression of the protective E1B region is driven bytk, i.e. a promoter that is not as strong as the mouse CMV promoter (butstronger than the native E1B promoter). Consistent with the protectiveeffect conferred by E1B far fewer foci in fewer cell samples wereobtained with this construct when compared to the results with plasmid49E.

Formation of foci with both primary cell appearance and transformedphenotype was also observed in cultures of liver transfected with CELOplasmids 36E (SEQ ID NO:19) and 25F (SEQ ID NO:10).

Cultures with foci were expanded by treatment with trypsin for 2-3 minand resuspension in DMEM:F12 medium for transfer to fresh culturevessels.

For cryopreservation at regular intervals cells were removed withtrypsin, resuspended in DMEM:F12 medium containing 10% FCS, collected bycentrifugation at 200×g for 10 min, resuspended in DMEM:F12 mediumcontaining 50% FCS and 10% DMSO (Sigma, USA) to a concentration ofapproximately 3×10⁶ cells per ml, and cooled with a rate of 1° C. permin to −80° C. After 24 hours, the cells were transferred to liquidnitrogen for permanent storage.

Example 3 Immunofluorescence Assay for Stable Transfection

Cultures of potentially immortalized cells were seeded on glass slidesand allowed to proliferate for several days before fixation withice-cold methanol for 10 min. The fixed cells were incubated withantibodies against E1A and E1B 55K proteins, condary antibodies, andfluorescent dye specific against the latter according to standardimmunofluorescene methods (Becton Dickinson, UK, #554155 antibodyagainst E1A, diluted 1:30; Oncogene, USA, #DP08-100UG antibody againstE1B 55K, diluted 1:30; secondary antibody directed against mouse or rat,respectively, and conjugated to biotin, both from Jackson ImmunoResearch, USA, diluted 1:80; visualization with Jackson Immuno Research,USA, #016-070-084 streptavidin-Texas Red conjugate, diluted 1:100).Primary cells still abundant in early, not yet fully establishedimmortalized cell lines and readily distinguishable by morphologyprovided a convenient internal negative control for antibodyspecificity. 293 cells (human embryonic kidney cells) that stablyexpress the Ad5 E1-region served as positive control. DAPI(4′,6-diamidino-2-phenylindol; Sigma, USA) to 1 μg/ml was added in thefinal incubation step to stain the nuclei of the cells for orientationpurposes.

A strong signal for E1A and 55K was observed only in cells thatunderwent characteristic changes in morphology confirming successfulimmortalization by the transfected plasmids (FIG. 3). Furthermore,spontaneous transformation, a formal possibility, was not observed asall cells with altered phenotype were E1-positive. None of the cellswith primary phenotype expressed E1-proteins. Although possible intransfections of supercoils where the linearization of plasmid in theprocess of integration occurs at random positions none of the examinedfoci exhibited E1A expression in absence of E1B expression, furtheremphasizing the requirement for dual pathway disruption forimmortalization.

Example 4 Assay for Endogenous and Exogenous Retroviruses

A common problem encountered when vaccines are produced in primarychicken fibroblasts is contamination with exogenous or endogenousretroviruses. The diversity of the retrovirus family is too complex topredict whether a given species is a carrier for retroviruses. Reportsfrom the literature therefore usually are limited to a subset of theretrovirus family, for example EAV-HP/ALV subgroup J (Smith, L. M. etal., J. Gen. Virol. 80(pt1):261-8 (1999)), and then only to a subset ofavian species.

A reliable confirmation of contamination with retroviruses thereforeshould focus on a common motif present in these viruses. Sequencediversity precludes nucleic acid-based detection methods. However,common to all retroviruses is the presence of the reverse transcriptaseenzyme. The supernatant of expanded foci from duck liver cellsimmortalized with plasmid 49E was therefore assayed by quantitativeprobe-based product enhanced PCR for reverse transcriptase (Q-PERT) andcompared to several controls, inter alia CHO as positive control and 293cells as negative control (see below and FIG. 4), to detect bothendogenous retroviral activity or contamination with free retroviruses.The assay is a modification from the literature (Lovatt, A. et al., J.Virol. Methods 82(2): 185-200 (1999)). Briefly: retroviruses wereenriched from culture supernatant by ultracentrifugation with 100000×gthrough a barrier of 20% sucrose in PBS to remove cellular debris.Virions (if present) were resuspended into lysis buffer (50 mM Tris pH7.8, 80 mM KCl, 2.5 mM DTT, 0.75 mM EDTA, 0.5% Triton X-100) and mixedwith substrate buffer (10 mM each of dATP, dCTP, dGTP, and dTTP; 15 μMspecific primer [GCC TTT GAG AGT TAC TCT TTG; SEQ ID NO:15]; and 0.5mg/ml fragmented herring sperm DNA [Promega Corp, #D1811]) containing amodel RNA (5 μg/ml Brome Mosaic Virus RNA [Promega Corp, USA, #D1541])that is reverse transcribed if RT activity is present in the sample.cDNA from the model RNA is amplified by PCR with primers (MA CAC TGT ACGGCA CCC GCA TT; SEQ ID NO:16) and (GCC UT GAG AGT TAC TCT TTG; SEQ IDNO:17) and detected via SYBR green fluorescence in an AB 7000 SequenceDetection System using the QPCR SYBR Green ROX Mix #AB-1163 from Abgene,UK, according to the instructions of the manufacturer.

FIG. 4 demonstrates strong RT activity in CHO cells as expected fromreports in the literature (for example, Anderson, K. P. et al., Virology181(1): 305-311 (1991)). With these cells as positive control and human293 cells free of retroviral activity as negative control a bracket isdefined that allows interpretation of unknown RT activity in thesupernatant of cell cultures (FIG. 4, bold squares and bold triangles).

We found moderate RT-activity in chicken embryo fibroblasts (FIG. 4,bold diamond symbols).

The signal for RT activity in the duck cell supernatant was congruentwith the signal for RT activity in 293 cells, and both again congruentwith a control representing the detection limit for our assay consistingof model RNA not incubated with RT (FIG. 4, compare curves with open andbold triangles and grey circles). Equivalent levels of signal intensity(delta Rn) were separated by at least two cycle numbers between samplesfrom CHO cells and chicken embryo fibroblasts (that for theseexperiments are derived from a source known to be only weaklyRT-positive) and by at least four cycle numbers between samples from CHOcells and the 293 negative control and the duck cell culture. Thus,contrary to chicken cells the described duck cells do not exhibit RTactivity and thus fulfill an essential attribute for suitability inpharmaceutical applications.

Example 5 Modified Vaccinia Virus Ankara (MVA)

Suitability of the expanded foci as substrate for amplification of MVAwas determined for liver, retina, somites and extra-embryonic membranelines. Table 1 and FIG. 5 show results obtained by infection of the celllines with an inoculum prepared from a large scale preparation of MVA(ATCC #VR-1508) on CEFp, primary chicken embryonic fibroblasts. The datain the table obtained by infection with an MOI (multiplicity ofinfection or number of infectious particles per host cell) of 0.1demonstrates that viral output of retina and somite cells (in plaqueforming units per ml) are comparable to or even exceed the outputobtained with CEFp cells.

TABLE 1 Comparison of virus titers obtained in parallel infections of 1to 5 × 10⁵ cells in cavities of 24-well plates. MVA yield in pfu/ml(after 48 h, infection with MOI of 0.1) CEFp 3.54 × 10⁷ retina 2.06 ×10⁷ liver 3.20 × 10⁴ somite 4.60 × 10⁷ membrane 4.03 × 10³ Input viruswas adjusted for an MOI of 0.1. CEFp, fresh primary chicken embryonicfibroblasts; membrane, extra-embryonic membrane.

Plaque-forming units for MVA on duck cells were determined as follows:MVA virus was recovered from infected cells after 48 hours from thesupernatant and from adherent cells opened by repeated freeze-thawing.VERO (African green monkey kidney) cells were seeded in 96 well plates(2×10⁴ cells per well) and infected with serial 10-fold dilutions ofMVA-containing suspension on the following day. Two days thereafter, thecultures were fixed with methanol and infected cells incubated withpolyclonal vaccinia virus antibodies (Quartett, Germany, #9503-2057, at1:1000 dilution in PBS containing 1% fetal calf serum) for 1 hour at 37°C. Two wash steps were performed with PBS containing 0.05% Tween 20(Sigma Corp, USA) and secondary antibody to the vaccinia-specificantibody is added at 1:1000 dilution in PBS containing 1% fetal calfserum. This secondary antibody is coupled to the peroxidase enzyme thatcatalyzes a color reaction upon incubation with AEC reagent(3-amino-9-ethyl-carbozole; 0.3 mg/ml in 0.1 M acetate buffer pH 5.0containing 0.015% F60₂). Infected foci are identified by lightmicroscopy and plaque forming units are calculated from the maximumdilution of MVA suspension that yields a positive dye reaction.

FIG. 5 depicts the output of virus per cell. The output of virus percell correlates with permissiveness of a given host cell for aparticular virus. Permissiveness is influenced by biochemical propertiessuch as receptor density or efficiency of processing of viral structuralproteins. FIG. 5 demonstrates that the number of infectious particlesreleased per retina cell or per somite cell compares favourably with theobtained infectious particles per chicken embryonic fibroblast.

Division of “output virus per cell” by the “MOI” yields the burst size,the ratio of input virus to output virus. Burst size is equivialent toamplification of virus and thus important to estimate cost and requiredresources for large scale production. The determined burst sizes in thedescribed example are 374 for CEFp, 513 for retina cells, and 1108 forsomite-derived cells. Retina cells and somite cells yield better valuesthan fresh primary chicken embryo fibroblasts and thus should providesuperior substrates for large scale production of MVA.

The unsatisfactory results for MVA amplification obtained with cellsderived from liver or extra-embryonic membrane cannot be extended toother virus families: it is evident to one familiar with the art thatamplification of other viruses, for example vaccine-relevant influenzaviruses; may be extremely successful on these cells.

It is conceivable that with subsequent passaging of virus on a givenhost cell the output titer decreases. Such events may occur if hostcells support most but not all steps in the various stages of theinfectious cycle. To address this question serial passage of MVA wasperformed on duck retina cells transformed with plasmid 49E. The data inFIG. 6 demonstrate that MVA is not lost with passaging on these cells:at similar levels of input virus adjusted to an MOI of 0.3 (given bybars in FIG. 6) the burst size (bold squares) increases nine-fold from35 to 315. The reason for the increase in burst size may be due to toimproved properties of the host cell as passage number increases.

In conclusion, duck retina and somite-derived cells obtained bytransfection of Ad5-E1 region under cGMP conditions stably supportamplification of MVA with an efficiency comparable to or better thanprimary chicken embryonic fibroblasts. Due to the highly attenuatednature of MVA conventional cell lines for large-scale production ofviruses are not suitable. It is a surprising finding that designed duckcell lines performed better than primary chicken cells in propagation ofMVA and thus are able to provide novel production platforms for thisimportant vaccine candidate. The described cell lines were generatedunder cGMP conditions and are therefore suitable for pharmaceuticalapplication.

1-14. (canceled)
 15. An avian cell line free of reverse transcriptaseactivity and permissive for modified vaccinia virus Ankara (MVA). 16.The cell line of claim 15, wherein the cell line is infected with andreplicates MVA.
 17. The cell line of claim 15, wherein the cell line isnot a neoplastic cell line derived from naturally occurring orspontaneous tumors.
 18. The cell line of claim 15, wherein the cell linedoes not release infectious virus particles from endogenousretroviruses.
 19. The cell line of claim 15, wherein the cell line doesnot release infectious virus particles from endogenous retroviruses, isinfected with MVA and replicates MVA.
 20. The cell line of claim 15,wherein the cells are immortalized.
 21. The cell line of claim 20,wherein the immortalized cells are derived from primary cells, cellsfrom isolated body segments or separated individual organs.
 22. The cellline of claim 15, further comprising a first gene encoding an expressionproduct capable of overcoming G1 checkpoint control and a second geneencoding an expression product capable of preventing apoptosis inducedby expression product of the first gene.
 23. The cell line of claim 15,further comprising an antitumor gene.
 24. The cell line of claim 22,wherein the expression product of the first gene is capable of mediatingdisruption of complexes between retinoblastoma proteins and E2Ftranscription factors; and/or wherein the expression product of thesecond gene is capable of preventing transcriptional activation by p53.25. The cell line of claim 15, which is immortalized with a combinationof viral and/or cellular genes (gene(s)), wherein the cell linecomprises a combination of viral and/or cellular genes including: atleast one first gene encoding an expression product capable of affectingthe function of the retinoblastoma protein by mediating disruption ofcomplexes between retinoblastoma proteins and E2F transcription factors;and at least one second gene encoding an expression product capable ofaffecting the p53 protein or a family member thereof; wherein the firstgene is a viral gene selected from the group consisting of an adenovirusE1A gene from mastadenoviruses, an E7 gene of papillomaviruses, an orf22 gene of avian adenoviruses, and E43 open reading frames from ovineattadenovirus; or is a cellular gene mediating disruption of complexesbetween retinoblastoma proteins and E2F transcription factors and beingselected from Cyclins D1, D2 and D3, and a mutated CDK4 not susceptibleto inactivation by p16INK4a; and wherein the second gene is a viral genecoding for a protein preventing induction of growth arrest and apoptosisby p53 selected from the group consisting of adenovirus E1B55K proteinof all groups, GAM-1 of chicken embryo lethal orphan virus CELO and E6protein of papillomaviruses; or is mdm2 being a cellular gene preventinggrowth arrest and apoptosis by p53.
 26. The cell line of claim 25,wherein the first gene is selected from the group consisting of an E1Agene from mastadenovirus of group C, and an E7 gene from a low-riskhuman papilloma virus (HPV) and/or wherein the second gene codes for anE6 protein from a low-risk HPV.
 27. The cell line of claim 15, which isimmortalized with (i) E1A and E1B encoding genes of an adenovirus fromthe genus Mastadenovirus; and/or (ii) orf22 and GAM-1 encoding genesfrom the genus aviadenovirus CELO.
 28. The cell line of claim 15,further comprising non-natural functional sequences selected from thegroup consisting of transgenes, promoters, enhancers and selectionmarkers.
 29. The cell line of claim 15, wherein the cell line is free ofanimal serum.
 30. The cell line of claim 15, wherein the cell line is aduck cell line.
 31. The cell line of claim 30, wherein the duck cellline is a cell line originating from duck somites, duck neuronal tissueor duck retina.
 32. The cell line of claim 15, wherein the cell line iscell line 12A07-A10 (DSM ACC 2695).
 33. A method for producing MVA,which comprises (i) contacting MVA with an avian cell line as defined inclaim 1; and (ii) cultivating said MVA on said avian cell line.
 34. Themethod of claim 33, wherein the avian cell line does not releaseinfectious virus particles from endogenous retroviruses.
 35. The methodof claim 33, wherein the avian cell line is a duck cell line.